Archive for the ‘Coding and Reversing’ Category

Introduction

It was more than six years ago that I first posted on the concept of protected processes, making my opinion of this poorly thought-out DRM scheme clear in the title alone: “Why Protected Processes Are A Bad Idea”. It appears that Microsoft took a long, hard look at the mechanism (granted, an impenetrable user-mode process can have interesting security benefits — if we can get DRM out of the picture), creating a new class of process yet again: the Protected Process Light, sometimes abbreviated PPL in the kernel.

Unlike its “heavy” brother, the protected process light actually serves as a type of security boundary, bringing in three useful mitigations and security enhancements to the Windows platform. Over the next three or four blog posts, we’ll see how each of these enhancements is implemented, starting this week with Pass-the-Hash (PTH) Mitigation.

We’ll talk about LSASS’ role in the Windows security model, followed by the technical details behind the new PPL model. And since it’s hard to cover any new security advancement without delving in at least a few other inter-related internals areas, we’ll also talk a little bit about Secure Boot and protected variables. Perhaps most importantly, we’ll also see how to actually enable the PtH mitigation, as it is currently disabled by default on non-RT Windows versions.

The LSASS Process

In Windows, local user accounts are hashed using a well-known algorithm (NTLM) and stored in a database called the SAM (Security Accounts Manager), which is in itself a registry hive file. Just like with other operating systems, a variety of offline, and online attacks exist in order to obtain, reset, or otherwise reuse the hashes that are stored in the SAM, going from the usual “Password Reset” boot emergency disks, to malicious privilege escalation. Additionally, a variety of other cryptographic data is also stored in the SECURITY database, yet another registry hive file. This data includes information such as secrets, saved plain-text passwords, and more.

A process called the Local Security Authority (LSASS) manages the run-time state of this information, and is ultimately responsible for all logon operations (including remote logon over Active Directory). Therefore, in order to obtain access to this data, two primary mechanisms are used:

1) File-based attacks: the SAM/SECURITY hives are accessed, either offline, or online through tricks such as using Volume Shadow Copies, and the hashes + secrets extracted. This mechanism has disadvantages in that the storage formats can change, detailed registry knowledge is needed, and LSASS will often obfuscate much of the data (such as plain-text cached passwords).

2) Process-based attacks: since the hash and secret data from #1 above is neatly loaded by LSASS in readable form (and accessible thanks to easy-to-use query APIs), it is often much more preferable to simply inject code into the LSASS process itself, which is then used to dump hashes or secrets, as well as to create tokens based on those hashes. Additionally, researchers such as Gentil Kiwi have even discovered that LSASS contains plain-text passwords using reversible symmetric cryptography (with the key stored in the LSASS process itself). Tools now exist today to not only pass-the-hash, but to also pass-the-pass. In a default Windows 8 installation, both the local user account password, as well as the Microsoft Live Services password, is available in a plaintext-retrievable way.

Obviously, both this file and the process are protected such that only the SYSTEM account can access them. But once running as Administrator, this is a simple hurdle — and since most users still run as Administrators (albeit with UAC, but that’s not a security boundary), exploits only have to escape whatever local sandbox they’re running in, get admin rights, get a system token, and inject into LSASS. And of course, in a shared computer environment, another admin on the machine can get the passwords of all the users.

What’s changed in Windows 8.1? Run Mimikatz or other pass-the-hash attacks and they still work out-of-the-box. But on a Windows 8.1 RT system (supposing one can compile for ARM), they won’t — in fact, even attempting to attach a debugger to the LSASS process will fail, regardless of user-mode permissions.

The title of this blog post gives it away: in Windows 8.1 RT, LSASS is now a protected process light. And with Registry Editor and the right key/value pair, your Windows 8.1 installation (non-RT) can take advantage of this too.

Protected Process Light Internals

Before taking a look at how to enable the mitigation, let’s see what makes a PPL tick. Unlike the simple “ProtectedProcess” bit in EPROCESS that I documented in Vista, a Windows 8.1 EPROCESS structure now has a “Protection” field of the following type:

Once a process is in the PPL state, all the protections in my previous blog post are in effect — the system protects both types of protected processes in the same way, preventing any handle open for all but a few limited rights. Additionally, the memory manager will prevent loading of DLLs that are not signed appropriately, using the Code Integrity improvements in Windows 8 that I covered in my talk at BreakPoint last year — something I plan to revisit in this blog at a later time.

Finally, although I didn’t mention this back in the Vista days, the application compatibility database is also disabled for these processes — an interesting attack vector that is blocked thanks to this.

Enabling the Pass-the-Hash Mitigation

Now that we know about this improvement to the security architecture, how can one take advantage of it on a non-RT Windows 8.1 computer ? By looking at the updated flow of Wininit.exe, the process in charge for launching LSASS, one can see that the ExecSystemProcessesroutine now calls GetLsaProtectionLevelwhich does a registry key read of HKLM\SYSTEM\CurrentControlSet\Control\Lsa for the value RunAsPPL. Before reading the registry however, it also calls ReadLsaConfigEnvironmentVariable— the importance of which we’ll see in a bit.

Either way, as long as one of these two things is set (the environment variable or the registry key), ExecSystemProcesseswill call StartSystemProcesswith the CREATE_PROTECTED_PROCESS flag. In turn, the routine will utilize the new Vista Process/Thread Attribute List functionality to add attribute 0x2000B — documented as the new Windows 8.1 “Protection Level Attribute“. As you can expect, the level is set to 4, which matches the “LSA Signer” enumeration value above. And just like that, LSASS is now a PPL, and protected against even an admin (or even SYSTEM) attacker. And no, not even SE_DEBUG_PRIVILEGE will get you through. Clicking on any of the linked function names will reveal Hex-Rays output to match this flow.

As a side note, is this all you need to launch as process as protected light — a protection level in a new attribute? Astute readers have probably already dumped the EPROCESS for Wininit.exe by now and noticed that, it too, is a PPL process (albeit, with a different Level!). The security model isn’t stupid — a PPL can only be launched by another PPL (or a PP, which is even more protected), and there’s a hierarchy in the levels as well, which we’ll see in a later post. Obviously, this means that Smss.exe (Wininit’s parent) must also be a PPL, and evidently the kernel has been running as a Protected Process since Vista. You could call this a user-mode protected chain of trust. These processes aren’t the only PPLs — we’ll see a lot more in a future post, and their purpose and configurability.

Should you run off and set that registry key? Yes and no. Once LSASS runs as a PPL, this will break any 3rd party software that might be attempting to inject or modify LSASS state. And sadly, at work, I’ve seen a number of these. Additionally, LSASS has a number of extensibility points, some used as ASEPs by attackers, others used legitimately to provide enhanced security or cryptographic services. Without the right signature and EKU (which right now means a WHQL signature with Microsoft as the signer — not just any Authenticode garbage!), those DLLs, plugins, and extensions will stop working. In certain IT scenarios, this can be a catastrophic compatibility problem, no doubt why Microsoft has chosen to keep this disabled for now.

But on a home computer, where you know you don’t have specialized software, and you firmly believe that AV (and others) should leave your LSASS alone? I’d say go for it. A number of helpful event log entries in the Security log will warn you of any DLLs that failed to load in case you’re curious.

Enhanced LSASS Mitigation With Secure Boot

Leaving the endless debate and controversy around Secure Boot aside, running Windows 8.1 on an UEFI-compatible machine with Secure Boot turned on will add an additional layer of security. Set the registry key as indicated above, reboot, watch LSASS run as a PPL, and now try deleting the registry key — then reboot again. LSASS will still run as a PPL. In fact, you can even re-install Windows 8.1, and LSASS will still run as a PPL. This is because Microsoft realized — if the attacker runs as Admin/SYSTEM and can inject into LSASS, but a registry key prevents this — why wouldn’t the Admin/SYSTEM attacker simply delete the key? Outside of active-key-monitoring shenanigans (which some parts of the kernel do employ, mostly licensing), not much. And definitely an offline attacker will have no problem editing the hive directly (unless BitLocker is also active).

This changes with Secure Boot however, as Windows has the ability to use the standard UEFI system variable runtime routines and set a value directly in the firmware store using SetFirmwareEnvironmentVariableEx API (and its kernel equivalents such as the NtSetSystemEnvironmentVariableEx and ExSetFirmwareEnvironmentVariable routines). To be fair, this is standard UEFI behavior; what Secure Boot brings to the table is the Namespace GUID that Windows can use — which if you were paying attention you saw in the ReadLsaConfigEnviromentVariable snippet earlier.

This GUID, {77FA9ABD-0359-4D32-60BD-28F4E78F784B}, is the “Protected Store” that Windows can use to store certain system properties it wants to protect. In this case, it stores a variable named Kernel_Lsa_Ppl_Config that is associated with the RunAsPPLvalue in the registry (to be 100% accurate, “it” here refers to Winload.efi, which upon loading the registry executes the OslFwProtectSecConfigVars routine) . As soon as this variable is set, the registry values no longer matter — PPL is enabled for LSASS.

What prevents a user from simply deleting this variable, or setting it to zero? Witness the following snippet in the NtSetSystemEnvironmentVariableEx system call, which executes for user-mode callers:

The intent is clear — any variables stored in the Secure Boot GUID, that start with Kernel_, are inaccessible from userspace — meaning that no Windows application can attempt to reset the protection. In fact, the only way to reset the protection is to boot into a special UEFI application written by Microsoft, which will wipe the environment variable based on the user’s input. An impressive security boundary, to say the least.

Conclusion

At the end of the day, what does running as PPL really mean for your system? Based on the limited access rights that protected processes (and PPLs) provide, a process, regardless of its token, can no longer open a handle for injection and/or modification permissions toward the LSASS process. Since this is critical for injecting the DLLs and/or threads that process-based PtH tools use, their use is thwarted. Additionally, attempts to load DLLs into LSASS through other means (such as AppInit_DLLs or LSA extensions) are also blocked, since the required digital signatures are missing. It’s important to mention that file-based hash attacks are not affected by these enhancements — at the end of the day, if someone has local console access to your unlocked, non-encrypted machine, it’s not your machine anymore.

With Windows 8.1, Protected Processes have evolved — taking on additional capabilities and now working to enhance security and protect users, instead of doing the bidding of the MPAA. One such new capability is the Pass-the-Hash mitigation and general hardening of the LSASS process — but there are a lot more. It’s one of the first of many general security and cryptographic enhancements in Windows 8.1 which provide additional boundaries around Microsoft’s code — separating it from other people’s code. But just like Apple’s entitlement system, it’s not a fully walled garden. Further posts will explore not only additional uses of PPLs by Windows’ own binaries, but also (supported) options available for 3rd parties.

Introduction

As some of you may know, back in June of 2013, I gave a talk at Recon, a security conference in Montreal, about KASLR Information Bypasses/Leaks in the Windows NT kernel, entitled “I got 99 problems but a kernel pointer ain’t one”. The point of the presentation was both to collect and catalog the many ways in which kernel pointers could be leaked to a local userspace attacker (some of which were known, others not so much), as well as raise awareness to the inadequate protection, and sometimes baffling leaking of, such data.

After sharing my slides and presentation with some colleagues from Microsoft, I was told to “expect some changes in Windows 8.1”. I was initially skeptical, because it seemed that local KASLR bypasses were not at the top of the security team’s list — having been left behind to accumulate for years (a much different state than Apple’s OS X kernel, which tries to take a very strong stance against leaking pointers). As Spender likes to point out, there will always be KASLR bugs. But in Windows, there were documented APIs to serve them on a platter for you.

Restricted Callers

Our investigation begins with an aptly named new Windows 8.1 kernel function:

This now introduces a new security term in the Windows kernel lingo — a “restricted caller”, is a caller whose integrity level is below Medium. For those unfamiliar with the concept of integrity levels, this includes most applications running in a sandbox, such as Protected Mode IE, Chrome, Adobe Reader and parts of Office. Additionally, in Windows 8 and higher, it includes all Modern/Metro/TIFKAM/MoSH/Immersive/Store applications.

So, what is it exactly that these restricted callers cannot do?

System-wide Information Mitigations

First of all, STATUS_ACCESS_DENIED is now returned when calling NtQuerySystemInformation, with the following classes:

SystemModuleInformation — Part of my (and many others) presentation, this disables the EnumSystemDrivers API and hides the load address of kernel drivers (finally!).

SystemModuleInformationEx — A new information class that was recently added in Vista and leaked as much as the one above.

SystemLocksInformation — Part of my presentation (and also found by j00ru), this leaked the address of ERESOURCE locks in the system.

SystemStackTraceInformation — Indirectly mentioned in the ETW/Performance section of my presentation, this leaked kernel stack addresses, but only if the right global flags were set.

SystemHandleInformation — Part of my presentation, and well known beforehand, this was NT’s KASLR-fail posterboy: leaking the kernel address of every object on the system that had at least one handle open (i.e.: pretty much all of them).

SystemExtendedHandleInformation — Another new Vista information class, which was added for 64-bit support, and leaked as much as above.

SystemObjectInformation — Part of my presentation, if the right global flags were set, this dumped the address of object types and objects on the system, even if no handles were open.

SystemBigPoolInformation — Part of my presentation, this dumped the address of all pool (kernel heap) allocations over 4KB (so-called “big” allocations).

SystemSessionBigPoolInformation — The session-specific little brother of the above, perfect for those win32k.sys exploits.

Thread Information Mitigations

But that’s not all! Using the well-known SystemProcessInformation information class, which famously dumps the entrypoint addresses of system threads (pretty much giving you a function pointer into almost all loaded drivers), as well as the kernel stack base and stack limit of all the threads on the system (used by j00ru in his GS-stack-cookie-guessing attacks, since the cookie is partly generated with this information), now introduces some additional checks.

First of all, there are now three information classes related to this data.

SystemProcessInformation, which is well-understood.

SystemExtendedProcessinformation, which was documented by j00ru and wj32. This returns the SYSTEM_EXTENDED_THREAD_ INFORMATION structure containing the stack base, limit, and Win32 start address.

SystemFullProcessInformation, which is new to Windows 8.1. This returns the SYSTEM_PROCESS_INFORMATION_EXTENSION below:

(By the way, I hear Microsoft is taking suggestions on the upcoming 4th information class in Windows 9. Current leader is SystemFullExtendedProcessInformation.)

It’s unfortunate that Microsoft continues to keep these APIs undocumented — the documented Win32 equivalents require up to 12 separate API calls, all of which return the same data 12 times, with the Win32 interface only picking one or two fields each time.

Back to our discussion about KASLR, the behavior of this information class is to also apply the restricted caller check. If the caller is restricted, then the stack limit, stack base, start address, and Win32 start address fields in the thread structures will all be zeroed out. Additionally, to use the new “full” information class, the caller must be part of the Administrators group, or have the Diagnostic Policy Service SID in its token. Interestingly, in these cases the restricted caller check is not done — which makes sense after all, as a Service or Admin process should not be running below medium integrity.

Process Information Mitigations

The checks for restricted callers do not stop here however. A few more interesting cases are protected, such as in NtQueryInformationProcess, in which ProcessHandleTracing is disabled for such callers. I must admit this is something I missed in my KASLR analysis (and no obvious hits appear on Google) — this is an Object Manager feature (ironically, one which I often use) related to !obtrace and global flags, which enables seeing a full stack trace and reference count analysis of every object that a process accesses. Obviously, enabling this feature on one own’s process would leak the kernel pointers of all objects, as well as stack traces of kernel code and drivers that are in the path of the access (or running in the context of the process and performing some object access, such as during an IRP).

Another obvious “d’oh!” moment was when seeing the check performed when setting up a Profile Object. Profile Objects are a little-talked about feature of NT, which primarily power the “kernrate” utility that is now rather deprecated (but still useful for analyzing drivers that are not ETW-friendly). This feature allows the caller to setup “buckets” — regions of memory — in which every time the processor is caught with its instruction pointer/program counter cause a trace record to be recorded. In a way similar to some of the cache/TLB prediction attacks shown recently, in which the processor’s trace buffer is queried for address hits, the same could be setup using an NT profile object, which would reveal kernel addresses. In Windows 8.1, attempts to setup buckets above the userspace barrier will result in failure if the caller is restricted.

Last but not least, the ProcessWorkingSetWatch and ProcessWorkingSetWatchEx classes of NtQueryInformationProcess are also now protected. I didn’t talk about these two at Recon, and again I’m not aware of any other public research on these, but they’ve always been my favorite — especially because PSAPI, documented on MSDN, exposes Win32 friendly versions of these (see GetWsChanges). Basically, once you’ve turned WS Watch on your process, you are given the address of every hard fault, as well as the instruction pointer/program counter at the time of the fault — making it a great way to extract both kernel data and code addresses. Instead of going through the trouble of pruning kernel accesses from the working set watch log, the interface is now simply completely disabled for restricted callers.

Conclusion

Well, there you have it folks! Although a number of undocumented interfaces and mechanisms still exist to query protected KASLR pointers, the attack surface has been greatly decreased — eliminating almost all non-privileged API calls, requiring at least Medium IL to use them (thus barring any Windows Store Apps from using them). This was great work done by the kernel security team at Microsoft, and continues to showcase the new lengths at which Windows is willing to go to maintain a heightened security posture. It’s only one of the many other exciting security changes in Windows 8.1

Which would indicate that Win8 now has a new kind of “ASSERT” statement that is present in retail builds, designed for checking again certain common security issues, such as corrupted/dangling list pointers.

Thankfully, Microsoft was nice enough to document where this is coming from, and I’ve even been told they want to encourage its use externally. Starting in “Windows 8”, if you leave NO_KERNEL_LIST_ENTRY_CHECKS undefined, the new LIST_ENTRY macros will add a line RtlpCheckListEntry(Entry); to verify the lists between operations. This expands to:

So there you have it, the new __fastfail intrinsic generates an INT 29H, at least on x86, and the preceding 8 security failures are registered by Windows — I assume driver developers and user application developers could define their own internal security codes as well, preferably starting with a high enough ID not to interfere with future codes Microsoft may choose to add.

The bugcheck, by the way, is defined as:

//// MessageId: KERNEL_SECURITY_CHECK_FAILURE//// MessageText://// A kernel component has corrupted a critical data structure.// The corruption could potentially allow a malicious user to// gain control of this machine.//#define KERNEL_SECURITY_CHECK_FAILURE ((ULONG)0x00000139L)

This is a great mechanism that should make security issues much more “visible” to users, even if it means taking the system down. Hopefully the new and improved blue screen of death — the Sad Face Of Sorrow (SFOS) — will give users more indication as to why their system had to be taken down, as the current implementation lacks the details needed to differentiate between a crash, and a security failure such as this.

This year I had the chance to present some security-related findings that I had made earlier during the year inside Win32k.sys, the Windows GUI Subsystem and Window Manager. I presented a total of four bugs, all local Denial of Service (DoS) attacks. Two of these attacks could be done on any system up to Vista SP0 (one up to Server 2008) from any account, while the other two were NULL-pointer dereferences that required administrative access to exploit (however, since they are NULL-pointer dereferences, the risk for user->kernel elevation exists in one of them).

Because the first two attacks (from any guest account) relied on highly internal behavior of the Windows NT Kernel (used in all of Microsoft’s modern operating systems today), I thought it was interesting to talk a bit about the internals themselves, and then present the actual bug, as well as some developer guidance on how to avoid hitting this bug.

I’d like to thank everyone who came to see the talk, and I really appreciated the questions and comments I received. I’ve also finally found out why Win32k functions are called “xxx” and “yyy”. Now I just need to find out about “zzz”. I’ll probably share this information in a later post, when I can talk more about Win32k internals.

As promised, you’ll find below a link to the slides. As for proof of concept code, it’s currently sitting on my home machine, so it’ll take another week. On the other hand, the slides make three of the bugs very clear in pseudo-code. I’ll re-iterate them at the bottom of the post.

One final matter — it seems I have been unable to make the bh08@alex-ionescu.com email work due to issues with my host. For the moment, please use another e-mail to contact me, such as my Google Mail account. My email address is the first letter of my first name (Alex) followed by my entire last name (Ionescu).

Bug 3:
1) Handle = CreateWindowStation(…)
2) Loop all handles in the process with the NtQuerySystemInformation API, or, just blindly loop every handle from 4 to 16 million and mark it as protected, or, use Process Explorer to see what the current Window Station handle is, and hard-code that for this run. The point is to find the handle to the current window station. Usually this will be a low number, typically under 0x50.
3) SetHandleInformation(CurrentWinstaHandle, ProtectFromClose)
4) SwitchProcessWindowStation(Handle);

In part 1 of the series, we covered some of the changes behind Vista’s new Session 0 Isolation and showcased the UI Detection Service. Now, we’ll look at the internals behind this compatibility mechanism and describe its behavior.

First of all, let’s take a look at the service itself — although its file name suggests the name “UI 0 Detection Service”, it actually registers itself with the Service Control Manager (SCM) as the “Interactive Services Detection” service. You can use the Services.msc MMC snap-in to take a look at the details behind the service, or use Sc.exe along with the service’s name (UI0Detect) to query its configuration. In both cases, you’ll notice the service is most probably “stopped” on your machine — as we’ve seen in Part 1, the service is actually started on demand.

But if the service isn’t actually running yet seems to catch windows appearing on Session 0 and automatically activate itself, what’s making it able to do this? The secret lies in the little-known Wls0wndh.dll, which was first pointed out by Scott Field from Microsoft as a really bad name for a DLL (due to its similarity to a malicious library name) during his Blackhat 2006 presentation. The version properties for the file mention “Session0 Viewer Window Hook DLL” so it’s probably safe to assume the filename stands for “WinLogon Session 0 Window Hook”. So who loads this DLL, and what is its purpose? This is where Wininit.exe comes into play.

Wininit.exe too is a component of the session 0 isolation done in Windows Vista — because session 0 is now unreachable through the standard logon process, there’s no need to bestow upon it a fully featured Winlogon.exe. On the other hand, Winlogon.exe has, over the ages, continued to accumulate more and more responsibility as a user-mode startup bootstrapper, doing tasks such as setting up the Local Security Authority Process (LSASS), the SCM, the Registry, starting the process responsible for extracting dump files from the page file, and more. These actions still need to be performed on Vista (and even more have been added), and this is now Wininit.exe’s role. By extension, Winlogon.exe now loses much of its non-logon related work, and becomes a more agile and lean logon application, instead of a poorly modularized module for all the user-mode initialization tasks required on Windows.

One of Wininit.exe’s new tasks (that is, tasks which weren’t simply grandfathered from Winlogon) is to register the Session 0 Viewer Window Hook Dll, which it does with an aptly named RegisterSession0ViewerWindowHookDll function, called as part of Wininit’s WinMain entrypoint. If the undocumented DisableS0Viewer value isn’t present in the HKLM\System\CurrentControlSet\Control\WinInit registry key, it attempts to load the aforementioned Wls0wndh.dll, then proceeds to query the address of the Session0ViewerWindowProcHook inside it. If all succeeds, it switches the thread desktop to the default desktop (session 0 has a default desktop and a Winlogon desktop, just like other sessions), and registers the routine as a window hook with the SetWindowsHookEx routine. Wininit.exe then continues with other startup tasks on the system.

I’ll assume readers are somewhat familiar with window hooks, but this DLL providesjust a standard run-of-the-mill systemwide hook routine, whose callback will be notified for each new window on the desktop. We now have a mechanism to understand how it’s possible for the UI Detection Service to automatically start itself up — clearly, the hook’s callback must be responsible! Let’s look for more clues.

Inside Session0ViewerWindowProcHook, the logic is quite simple: a check is made on whether or not this is a WM_SHOWWINDOW window message, which signals the arrival of a new window on the desktop. If it is, the DLL first checks for the $$$UI0Background window name, and bails out if this window already exists. This window name is created by the service once it’s already started up, and the point of this check is to avoid attempting to start the service twice.

The second check that’s done is how much time has passed since the last attempt to start the service — because more than a window could appear on the screen during the time the UI Detection Service is still starting up, the DLL tries to avoid sending multiple startup requests.
If it’s been less than 300 seconds, the DLL will not start the service again.

Finally, if all the checks succeed, a work item is queued using the thread pool APIs with the StartUI0DetectThreadProc callback as the thread start address. This routine has a very simple job: open a handle to the SCM, open a handle to the UI0Detect service, and call StartService to instruct the SCM to start it.

This concludes all the work performed by the DLL — there’s no other code apart from these two routines, and the DLL simply acknowledges window hook callbacks as they come through in cases where it doesn’t need to start the service. Since the service is now started, we’ll turn our attention to the actual module responsible for it — UI0Detect.exe.

Because UI0Detect.exe handles both the user (client) and service part of the process, it operates in two modes. In the first mode, it behaves as a typical Windows service, and registers itself with the SCM. In the second mode, it realizes that is has been started up on a logon session, and enters client mode. Let’s start by looking at the service functionality.

If UI0Detect.exe isn’t started with a command-line, then this means the SCM has started it at the request of the Window Hook DLL. The service will proceed to call StartServiceCtrlDispatcher with the ServiceStart routine specific to it. The service first does some validation to make sure it’s running on the correct WinSta0\Default windowstation and desktop and then notifies the SCM of success or failure.

Once it’s done talking to the SCM, it calls an internal function, SetupMainWindows, which registers the $$$UI0Background class and the Shell0 Window window. Because this is supposed to be the main “shell”
window that the user will be interacting with on the service desktop (other than the third-party or legacy service), this window is also registered as the current Shell and Taskbar window through the SetShellWindow and SetTaskmanWindow APIs. Because of this, when Explorer.exe is actually started up through the trick mentioned in Part 1, you’ll notice certain irregularities — such as the task bar disappearing at times. These are caused because Explorer hasn’t been able to properly register itself as the shell (the same APIs will fail when Explorer calls them). Once the window is created and a handler is setup (BackgroundWndProc), the new Vista “Task” Dialog is created and shown, after which the service enters a typical GetMessage/DispatchMessage window message handling loop.

Let’s now turn our attention to BackgroundWndProc, since this is where the main initialization and behavioral tasks for the service will occur. As soon as the WM_CREATE message comes through, UI0Detect will use the RegisterWindowMessage API with the special SHELLHOOK parameter, so that it can receive certain shell notification messages. It then initializes its list of top level windows, and registers for new session creation notifications from the Terminal Services service. Finally, it calls SetupSharedMemory to initialize a section it will use to communicate with the UI0Detect processes in “client” mode (more on this later), and calls EnumWindows to enumerate all the windows on the session 0 desktop.

Another message that this window handler receives is WM_DESTROY, which, as its name implies, unregisters the session notification, destroys the window lists, and quits.

This procedure also receives the WM_WTSSESSION_CHANGE messages that it registered for session notifications. The service listens either for remote or local session creates, or for disconnections. When a new session has been created, it requests the resolution of the new virtual screen, so that it knows where and how to display its own window. This functionality exists to detect when a switch to session 0 has actually been made.

Apart from these three messages, the main “worker” message for the window handler is WM_TIMER. All the events we’ve seen until now call SetTimer with a different parameter in order to generate some sort of notification. These notifications are then parsed by the window procedure, either to handle returning back to the user’s session, to measure whether or not there’s been any input in session 0 lately, as well as to handle dynamic window create and destroy operations.

Let’s see what happens during these two operations. The first time that window creation is being acted upon is during the afore-mentionned EnumWindows call, which generates the initial list of windows. The function only looks for windows without a parent, meaning top-level windows, and calls OnTopLevelWindowCreation to analyze the window. This analysis consists of querying the owner of the window, getting its PID, and then querying module information about this process. The version information is also extracted, in order to get the company name. Finally, the window is added to a list of tracked windows, and a global count variable is incremented.

This module and version information goes into the shared memory section we briefly described above, so let’s look into it deeper. It’s created by SetupSharedMemory, and actually generates two handles. The first handle is created with SECTION_MAP_WRITE access, and is saved internally for write access. The handle is then duplicated with SECTION_MAP_READ access, and this handle will be used by the client.

What’s in this structure? The type name is UI0_SHARED_INFO, and it contains several informational fields: some flags, the number of windows detected on the session 0 desktop, and, more importantly, the module, window, and version information for each of those windows. The function we just described earlier (OnTopLevelWindowCreation) uses this structure to save all the information it detects, and the OnTopLevelWindowDestroy does the opposite.

Now that we understand how the service works, what’s next? Well, one of the timer-related messages that the window procedure is responsible for will check whether or not the count of top level windows is greater than 0. If it is, then a call to CreateClients is made, along with the handle to the read-only memory mapped section. This routine will call the WinStationGetSessionIds API and enumerate through each ID, calling the CreateClient routine. Logically, this is so that each active user session can get the notification.

Finally, what happens in CreateClient is pretty straightforward: the WTSQueryUserToken and ImpersonateLoggedOnUser APIs are used to get an impersonation logon token corresponding to the user on the session ID given, and a command line for UI0Detect is built, which contains the handle of the read-only memory mapped section. Ultimately, CreateProcessAsUser is called, resulting in the UI0Detect process being created on that Session ID, running with the user’s credentials.

What happens next will depend on user interaction with the client, as the service will continue to behave according to the rules we’ve already described, detecting new windows as they’re being created, and detection destroyed windows as well, and waiting for a notification that someone has logged into session 0.

We’ll follow up on the client-mode behavior on Part 3, where we’ll also look at the UI0_SHARED_INFO structure and an access validation flaw which will allow us to spoof the client dialog.

One of the many exciting changes in Windows Vista’s service security hardening mechanisms (which have been aptly explained and documented in multipleblogs and whitepapers , so I’ll refrain from rehashing old material) is Session 0 Isolation. I’ve thought it would be useful to talk about this change and describe the behaviour and implementation of the UI Detection Service (UI0Detect), an important part of the infrastructure in terms of providing compatible behaviour with earlier versions of Windows.

As a brief refresher or introduction to Session 0 Isolation, let’s remember how services used to work on previous versions of Windows: you could run them under various accounts (the most common being System, Local Service and Network Service), and they ran in the same session the console user, which was logged-on to session 0 as well. Services were not supposed to display GUIs, but, if they really had to, they could be marked as interactive, meaning that they could display windows on the interactive window station for session 0.

Windows implemented this by allowing such services to connect to the Winsta0 Windowstation , which is the default interactive Windowstation for the current session — unlike non-interactive services, which belonged to a special “Service-0x0-xxx$” Windowstation, where xxx was a logon session identifer (you can look at the WDK header ntifs.h for a list of the built-in account identifiers (LUIDs)). You can see the existence of these windowstations by enumerating them in the object manager namespace with a tool such as Sysinternals’ WinObj.

Essentially, this meant three things: applications could either do Denial of Service attacks against named objects that the service would expect to own and create, they could feed malicious data to objects such as sections which were incorrectly secured or trusted by the service, and , for interactive services, they could also attempt shatter attacks — sending window messages with executable payloads in their buffer, exploting service bugs and causing the payload code to execute at higher privileges.

Session 0 Isolation puts an end to all that, by first having a separate session for the console user (any user session starts at 1, thus protecting named objects), and second, by disabling support for interactive services — that is, even though services may still display a UI, it won’t appear on any user’s desktop (instead, it will appear on the default desktop of the session 0 interactive windowstation).

That’s all fine and dandy for protecting the objects, but what if the service hasn’t been recompiled not to directly show a UI (but to instead use a secondary process started with CreateProcessAsUser, or to use the WTSSendMessage API) and depends on user input before continuing? Having a dialog box on the session 0 desktop without the user’s awareness would potentially have significant application compatibility issues — this is where the UI Detection Service comes into play.

If you’re like most Vista users, you’ve actually probably never seen the default desktop on session 0’s interactive windowstation in your life (or in simpler words, never “logged-on” or “switched to” session 0)! Since you can’t log on to it, and since interactive services which displayed UIs are thankfully rare, it remains a hidden mystery of Windows Vista, unless you’ve encountered such a service. So let’s follow Alice down the rabbit hole into session 0 wonderland, with some simple Service Controller (Sc.exe) commands to create our very own interactive service.

Using an elevated command prompt, create a service called RabbitHole with the following command:

Be careful to get the right spaces — there’s a space after each equal sign! You should expect to get a warning from Sc.exe, notifying you of changes in Windows Vista and later (the ones I’ve just described).

Now let’s start the service, and see what happens:

sc start RabbitHole

If all went well, Sc.exe should appear as if it’s waiting on the command to complete, and a new window should appear on your taskbar (it does not appear in the foreground). That window is a notification from the UI Detection Service, the main protagonist of this story.

Get ready to click on “Show me the Message” as soon as you can! Starting an essentialy fake service through Sc.exe will eventually annoy the Service Control Manager (SCM), causing it to kill notepad behind your back (don’t worry if this happens, just use the scstartRabbitHole command again).

You should now be in Session 0 (and probably unable to read the continuation of this blog, in which case the author hopes you’ve been able to find your way back!) As you can notice, Session 0 is a rather deserted place, due to the lack of any sort of shell or even the Theme service, creating a Windows 2000-like look that may bring back tears of joy (or agony) to the more nostalgic of users.

On the other hand, this desolate session it does contain our Notepad, which you should’ve seen disappear if you stayed long enough — that would be the SCM reaching its timeout of how long it’s willing to wait around hoping for Notepad to send a “service start” message back (which it never will).

Note that you can’t start any program you want on Session 0 — Cmd.exe and Explorer.exe are examples of programs that for one reason or another won’t accept to be loaded this way. However, if you’re quick enough, you can use an old trick common to getting around early 90ies “sandbox” security applications found in many libraries and elementary schools — use the common dialog control (from File, Open) to browse executable files (switch the file type to *.*, All Files), go to the System32 folder, right-click on Explorer.exe, and select Open. Notepad will now spawn the shell, and even if the SCM kills Notepad, it will remain active — feel free to browse around (try to be careful not to browse around in IE too much, you are running with System privileges!)

That’s it for this introduction to this series. In part 2, we’ll look at what makes this service tick, and in part 3, we’ll look at a technique for spoofing the dialog to lie to the user about which service is actually requesting input. For now, let’s delete the RabbitHole, unless you want to keep it around for impressing your colleagues:

Today I want to introduce another utility for Vista and Windows Server 2008 called ScTagQuery (short for Service Controller Tag Query), a tool which will allow you identify to which running service a certain thread inside a service hosting process (e.g Svchost.exe) belongs to, in order to help with identifying which services may be using up your CPU, or to better understand the stack trace of a service thread.

If you’ve ever had to deal with a service process on your system taking up too much CPU usage, memory, or other kinds of resources, you’ll know that Task Manager isn’t particularly helpful at finding the offending service because it only lists which services are running inside which service hosting process, but not which ones are consuming CPU time (in fact, some Svchosts host more than a dozen services!)

Process Explorer can also display this information, as well as the names of the DLLs containing the services themselves. Combined with the ability to display threads inside processes, including their starting address (which for some service threads, identifies the service they are associated with by corresponding to their service DLL) and call stack (which helps identify additional threads that started in a generic function but entered a service-specific DLL), Process Explorer makes it much easier to map each thread to its respective service name. With the cycles delta column, the actual thread causing high CPU usage can reliably be mapped to the broken or busy service (in the case of other high resource usage or memory leaks, more work with WinDBG and other tools may be required in order to look at thread stacks and contexts).

However, with each newer Windows release, the usage of worker pool threads, worker COM or RPC communication threads, and other generic worker threads has steadily increased (this is a good thing â€“ it makes services more scalable and increases performance in multiprocessor environments), making this technique a lot less useful. These threads belong to their parent wrapper routines (such as those in ole32.dll or rpcrt4.dll) and cannot be mapped to a service by looking at the start address. The two screenshots below represent a service hosting process on my system which contains 6 services — yet only one of the DLLs identified by Process Explorer is visible in the list of threads inside the process (netprofm.dll).

Again, there’s nothing wrong with using worker threads as part of your service, but it simply makes identifying the owner of the actual code doing the work a lot harder. To solve this need, Windows Vista adds a new piece of information called the Service Tag. This tag is contained in the TEB of every thread (internally called the Sub-Process Tag), and is currently used in threads owned by service processes as a way to link them with their owning service name.

When each service is registered on a machine running Windows Vista or later, the Service Control Manager (SCM) assigns a unique numeric tag to the service (in ascending order). Then, at service creation time, the tag is assigned to the TEB of the main service thread. This tag will then be propagated to every thread created by the main service thread. For example, if the Foo service thread creates an RPC worker thread (note: RPC worker threads donâ€™t use the thread pool mechanism â€“ more on that later), that thread will have the Service Tag of the Foo service.

ScTagQuery uses this Sub-Process Tag as well as the SCM’s tag database and undocumented Query Mapping API (I_ScQueryTagInformation) to display useful information in tracking down rogue services, or simply to glean a better understanding of the system’s behavior. Letâ€™s look at the option it supports and some scenarios where they would be useful.

During a live session, the -p, -s and -d options are most useful. The former will display a list of all service threads which have a service tag inside the given process, then map that tag to the service name. It can also function without a process ID, causing it to display system-wide data (enumerating each active process). The -s option, on the other hand, dumps which service tags are active for the process, but not the actual threads linked to them — it then links these tags to their service name. Finally, the -d option takes a DLL name and PID and displays which services are referencing it. This is useful when a thread running code inside a DLL doesn’t have an associated service tag, but the SCM does know which service is using it.

The -a and -n options are particularly useful when youâ€™ve obtained a service tag from looking at a crash dump yourself, and run the tool on the same system after a reboot. The -t option will let you map the service name if you know the PID of the service. If the PID changed or is otherwise unrecoverable, the -a option will dump the entire SCM tag database, which includes services which are stopped at the moment. Because these mappings are persistent across reboots, you’ll be able to map the thread with the service this way.

On the other hand, if all you’re dealing with is one thread, and you want to associate it to its service, the -t option lets you do just that.

Back to that same svchost.exe we were looking at with Process Explorer, here are some screenshots of ScTagQuery identifying the service running code inside rpcrt4.dll, as well as the service referencing the fundisc.dll module.

Note that Thread Pool worker threads do not have a Service Tag associated to them, so the current version of ScTagQuery on Vista cannot yet identify a service running code inside a worker pool thread inside ntdll.dll.

Finally, I should mention that ScTagQuery isnâ€™t the only tool which uses Service Tags to help with troubleshooting and system monitoring: Netstat, the tool which displays the state of TCP/IP connections on a local machine, has also been improved to use servicre tags to better identify who owns an open port (Netstat uses new information returned by various Iphlpapi.dll APIs which now store the service tag of every new connection). With the -b option, Netstat is now able to display the actual service name which owns an active connection, and not just the process name (which would likely just be svchost.exe).

The tool page for ScTagQuery is located here. You can download ScTagQuery in both 32-bit and 64-bit versions from this link. Windows Vista or higher is required.

After my departure from the ReactOSÂ project and subsequent new work for David Solomon, it wasn’t clear how much research and development on Windows internals I would still be able to do on a daily basis. Thankfully, I haven’t given up my number one passion — innovating, pushing the boundaries of internals knowledge, and educating users through utilities and applications.

In this vein, I have been working during my spare time on various new utilities that use new undocumented APIs and expose the internals behind Windows Vista to discover more about how the operating system works, as well as to be able to provide useful information to administrators, developers, students, and anyone else in between. In this post, I want to introduce my latest tool, MemInfo. I’ll show you how MemInfo can help you find bad memory modules (RAM sticks) on your system, track down memory leaks and even assist in detecting rootkits!

One of the major new features present in Windows Vista is Superfetch. Mark Russinovich did an excellent writeup on this as part of his series on Windows Vista Kernel Changes in TechNet Magazine. Because Superfetch’s profiling and management does not occur at the kernel layer (but rather as a service, by design choice), there had to be a new system call to communicate with the parts of Superfetch that do live in the kernel, just like Windows XP’s prefetcher code, so that the user-mode service could request information as well as send commands on operations to be performed.

Here’s an image of Process Explorer identifying the Superfetch service inside one of the service hosting processes.

Because Superfetch goes much deeper than the simple file-based prefetching Windows XP and later offer, it requires knowledge of information such as the internal memory manager lists, page counts and usage of pages on the system, memory range information, and more. The new SuperfetchInformationClass added to NtQuery/SetInformationSystem provides this data, and much more.

MemInfo uses this API to query three kinds of information:

a list of physical address ranges on the system, which describe the system memory available to Windows

information about each page on the system (including its location on the memory manager lists, its usage, and owning process, if any)

a list system/session-wide process information to correlate a process’ image name with its kernel-mode objectÂ

MemInfo ultimately provides this information to the user through a variety of command line options described when you run the utility. Some of its various uses include:

Seeing how exactly Windows is manipulating your memory, by looking at the page list summaries.

The Windows memory manager puts every page on the system on one of the many page lists that it manages (i.e. the standby list, the zeroed page list, etc). Windows Internals covers these lists and usage in detail, and MemInfo is capable of showing their sizes to you (including pages which are marked Active, meaning currently in-use and by the operating system and occupying physical memory (such as working sets) and not on any of the lists). This information can help you answer questions such as “Am I making good use of my memory?” or “Do I have damaged RAM modules installed?”.

For example, because Windows includes a bad page list where it stores pages that have failed internal consistency checks, MemInfo is an easy way (but not 100% foolproof, since Windowsâ€™ internal checks might not have detected the RAM is bad) to verify if any memory hardware on the system is misbehaving. Look for signs such as a highly elevated count of pages in the zeroed page list (after a day’s worth of computer use) to spot if Windows hasn’t been fully using your RAM to its potential (you may have too much!) or to detect a large memory deallocation by a process (which implies large allocations previously done).

Windows Vista also includes a new memory manager optimization called prioritized standby lists — the standby state is the state in which pages find themselves when they have been cached by Windows (various mechanisms are responsible for this of caching, including the cache manager and Superfetch) and are not currently active in memory. Mark covered these new lists in his excellent article as well.

To expose this information to system administrators, three new performance counters were added to Windows, displaying the size of the prioritized standby lists in groupings: priorities 0 through 3 are called Standby Cache Reserve, 4 and 5 are called Standby Cache Normal Priority, and finally, 6 and 7 are called Standby Cache Core. MemInfo on the other hand, which can also display these new lists, is an even better tool to identify memory in the standby state, since it is able to display the size of these lists individually.

While memory allocations on Windows XP (which could be part of application startup, the kernel-mode heap, or simple memory allocations coming from various processes) would consume pages from a single standby list and thus possibly steal away pages that more critical processes would’ve liked to have on standby, Windows Vista adds 8 prioritized lists, so that critical pages can be separated from less important pages and nearly useless pages. This way, when pages are needed for an allocation, the lower priority standby lists are used first (a process called repurposing). By making snapshots of MemInfo’s output over a period of time, you can easily see this behavior.

Here’s MemInfo output before, during, and after a large allocation of process private memory. Notice how initially, the bulk of my memory was cached on the standby lists. Most of the memory then became Active due to the ongoing large allocation, emptying the standby lists, starting by the lowest priority. Finally, after the memory was freed, most of the memory now went on the zero page list (meaning the system just had to zero 1GB+ of data).

Seeing to what use are your pages being put to by Windows

Apart from their location on one of the page lists, Windows also tracks the usage of each page on the system. The full list includes about a dozen usages, ranging from non-paged pool to private process allocations to kernel stacks. MemInfo shows you the partitioning of all your pages according to their usage, which can help pinpoint memory leaks. High page counts in the driver locked pages, non-paged pool pages and/or kernel stack pages could be indicative of abnormal system behavior.

The first two are critical resources on the system (much information is available on the Internet for tracking down pool leaks), while the latter is typically tightly maintained for each thread, so a large number may indicate leaked threads. Other usages should also expect to see a lower number of pages than ones like process private pages, which is usually the largest of the group.

At the time of this writing, here’s how Windows is using my 4GB of memory:

Looking at per-process memory usage, and detecting hidden processes

Internally, Windows associates private process pages with the kernel executive object that represents processes as managed by the process manager — the EPROCESS structure. When querying information about pages, the API mentioned earlier returns EPROCESS pointers — not something very usable from user-mode! However, another usage of this API is to query the internal list of processes that Superfetch’s kernel-mode component manages. This list not only allows to take a look at how much memory, exactly, belongs to each process on the system, but also to detect some forms of hidden processes!

Hidden processes are usually the cause of two things. The first is processes which have been terminated, but not yet fully cleaned up by the kernel, because of handles which are still open to them. Task Manager and Process Explorer will not show these processes, but MemInfo is the only tool apart from the kernel debugger which can (so you donâ€™t have to reboot in debugging mode). See below on how MemInfo is showing a SndVol32.exe process, created by Windows Explorer when clicking on the speaker icon in the system tray — Explorer has a bug which leaks the handles, so the process is never fully deleted until Explorer is closed.

The second cause of why a process may be hidden is a rootkit that’s hooking various system calls and modifying the information returned to user-mode to hide a certain process name. More advanced rootkits will edit the actual system lists that the process manager maintains (as well as try to intercept any alternate methods that rootkit detection applications may use), but MemInfo adds a new twist, by using a totally new and undocumented Superfetch interface in Windows Vista. It’s likely that no rootkit in the wild currently knows about Superfetch’s own process database, so MemInfo may reveal previously hidden processes on your system. Unfortunately, as with all information, it’s only a matter of time until new rootkits adapt to this new API, so expect this to be obsolete in the next two years.

There’s many more uses for MemInfo that I’m sure you can find — including as a much faster replacement for !memusage 8 if you’ve used that command in WinDBG before. MemInfo is fully compatible with both 32-bit and 64-bit versions of Windows Vista (including SP1 and Windows Server 2008 RC1 â€“ even though Server 2008 does not include Superfetch, because both client and server versions of the OS will use the same kernel as of Vista SP1, the API set is identical), but not any earlier version of Windows. Apart from these simple summary views, MemInfo is powerful enough to dump the entire database of pages on your system, with detailed information on each — valuable information for anyone that needs to deal with this kind of data. Furthermore, unlike using WinDBG to attach to the local kernel, it doesn’t require booting the system into debug mode.

You can download a .zip file containing both versions from this link. Make sure to run MemInfo in an elevated command prompt — since it does require administrative privileges. The documentation for MemInfo is located on the following page (this page is part of an upcoming website on which I plan to organize and offer help/links to my tools and articles).

The era of 64-bit computing is finally upon the consumer market, and what was once a rare hardware architecture has become the latest commodity in today’s processors. 64-bit processors promise not only a larger amount of registers and internal optimizations, but, perhaps most importantly, access to a full 64-bit address space, increasing the maximum number of addressable memory from 32-bits to 64-bits, or from 4GB to 16EB (Exabytes, about 17 billion GBs). Although previous solutions such as PAE enlarged the physically addressable limit to 36-bits, they were architectural “patches” and not real solutions for increasing the memory capabilities of hungry workloads or applications.

Although 16EB is a copious amount of memory, today’s computers, as well as tomorrow’s foreseeable machines (at least in the consumer market) are not yet close to requiring support for that much memory. For these reasons, as well as to simplify current chip architecture, the AMD64 specification (which Intel used for its own implementation of x64 processors, but not Itanium) currently only supports 48 bits of virtual address space — requiring all other 16 bits to be set to the same value as the “valid” or “implemented” bits, resulting in canonical addresses: the bottom half of the address space starts at 0x0000000000000000 with only 12 of those zeroes being part of an actual address (resulting in an end at 0x00007FFFFFFFFFFF), while the top half of the address spaceÂ starts at 0xFFFF800000000000, ending at 0xFFFFFFFFFFFFFFFF.

As you can realize, as newer processors support more of the addressing bits, the lower-half of memory will expand upward, towards 0x7FFFFFFFFFFFFFFF, while the upper-half of memory will expand downward, toward 0x8000000000000000 (a similar split to today’s memory space, but with 32 more bits). Anyone working with 64-bit code ought to be very familiar with this implementation, since it can have subtle effects on code when the number of implemented bits will grow. Even in the 32-bit world, a number of Windows applications (including system code in Windows itself) assume the most significant bit is zero and use it as a flag — clearly the address would become kernel-mode, so the application would mask this bit off when using it as an address. Now developers get a shot atÂ 16 bits to abuse as flags, sequence numbers and other optimizations that the CPU won’t even know about (on current x64 processors), on top of the usual bits that can be assumed due to alignment or user vs kernel-mode code location. Compiling the 64-bit application for Itanium and testing it would reveal such bugs, but this is beyond the testing capabilities of most developers.

Examples within Microsoft’s Windows are prevalent — pushlocks, fast references, Patchguard DPC contexts, and singly-linked lists are only some of the common Windows mechanisms which utilize bits within a pointer for non-addressing purposes. It is the latter of these which is of interest to us, due to the memory addressing limit it imposed on Windows x64 due to a lack of a CPU instruction (in the initial x64 processors) that the implementation required. First, let’s have a look at the data structure and functionality on 32-bits. If you’re unsure on what exactly a singly-linked list is, I suggest a quick read in an algorithm book or Google.

Here is the SLIST_HEADER, the data structure Windows uses to represent an entry inside the list:

Here we have an 8-byte structure, guaranteed to be aligned as such, composed of three elements: the pointer to the next entry (32-bits, or 4 bytes), and depth and sequence numbers, each 16-bits (or 2 bytes). Striving to create lock-free push and pop operations, the developers realized that they could make use of an instruction present on Pentium processors or higher — CMPXCHG8B (Compare and Exchange 8 bytes). This instruction allows the atomic modification of 8 bytes of data, which typically, on a 486, would’ve required two operations (and thus subjected these operations to race conditions requiring a spinlock). By using this native CPU instruction, which also supports the LOCK prefix (guaranteeing atomicity on a multi-processor system), the need for a spinlock is eliminated, and all operations on the list become lock-free (increasing speed).

On 64-bit computers, addresses areÂ 64-bits, so the pointer to the next entry must be 64-bits. If we keep the depth and sequence numbers within the same parameters, we require a way to modify at minimum 64+32 bits of data — or better yet, 128. Unfortunately, the first processors did not implement the essential CMPXCHG16B instruction to allow this. The developers had to find a variety of clever ways to squeeze as much information as possible into only 64-bits, which was the most they could modify atomically at once. The 64-bit SLIST_HEADER was born:

The first sacrifice to make was to reduce the space for the sequence number to 9 bits instead of 16 bits, reducing the maximum sequence number the list could achieve. This still only left 39 bits for the pointer — a mediocre improvement over 32 bits. By forcing the structure to be 16-byte aligned when allocated, 4 more bits could be won, since the bottom bits could now always be assumed to be 0. This gives us 43-bits for addresses — we can still do better. Because the implementation of linked-lists is used *either* in kernel-mode or user-mode, but cannot be used across address spaces, the top bit can be ignored, just as on 32-bit machines: the code will assume the address to be kernel-mode if called in kernel-mode, and vice-versa. This allows us to address up to 44-bits of memory in the NextEntry pointer, and is the defining constraint of Windows’ addressing limit.

44 bits is nothing to laugh at — they allow 16TB of memory to be described, and thus splits Windows into somewhat two even chunks of 8TB for user-mode and kernel-mode memory. Nevertheless, this is still 16 times smaller then the CPU’s own limit (48 bits is 256TB), and even farther still from the maximum 64-bits. So, with scalability in mind, there do exist some other bits in the SLIST_HEADER which define the type of header that is being dealt with — because yes, there is an official 16-bit header, written for the day when x64 CPUs would support 128-bit Compare and Exchange (which they now do). First, a look at the full 8-byte header:

Notice how the “HeaderType” bit is overlaid with the Depth bits, and allows the implementation and developers to deal with 16-byte headers whenever they will be put into use. This is what they look like:

Note how the NextEntry pointer has now become 60-bits, and because the structure is still 16-byte aligned, with the 4 free bits, leads to the full 64-bits being addressable. As for supporting both these headers and giving the full address-space to compatible CPUs, one could probably expect Windows to use a runtime “hack” similar to the “486 Compatibility Spinlock” used in the old days of NT, when CMPXCHG8B couldn’t always be assumed to be present (although Intel implemented it on the 586 Pentium, Cyrix did not). As of now, I’m not aware of this 16-byte header being used.

So there you have it — an important lesson not only on Windows 64-bit programming, but also on the importance of thinking ahead when making potentially non-scalable design decisions. Windows did a good job at maintaining capability and still allowing expansion, but the consequences of attempting to use parts of the non-implemented bits in current CPUs as secondary data may be hard to detect once your software evolves to those platforms — tread carefully.

Two weeks ago, I posted and published about a tool I wrote called Purple Pill, which used a bug in the ATI Vista x64 Video Driver to allow unsigned drivers to load. Within an hour, I pulled off the tool and the post, on my own accord, due to the fact I discovered ATI was not made aware of this particular flaw, and I wanted to follow responsible disclosure and allow ATI to fix their driver.

This flaw was especially severe, as it could be used for other purposes — including allowing a guest user access to a SYSTEM token, if the user was using a computer with an ATI Video Card. Therefore, it could have significant security implications beyond my original goal of bypassing Driver Signing on 64-bit Vista. On systems without the driver, administrative privileges would be required for any kind of attack. I originally thought this flaw had been reported to ATI since it was disclosed publically at Blackhat — something that’s usually only done once the presenter has notified the company. In this case, it seems this was not done, and I made an incorrect assumption. I should’ve checked the status of the flaw on my own, before posting the tool and I apologize for not having done so.

As for the act of bypassing driver signing, while I still disagree with Microsoft’s policy of not allowing users to set a permanent policy to explicitly allow this (at their own risk, perhaps even voiding support/warranty options), I have come to realize that attacking this policy isn’t the way to go. Microsoft has decided that Code Integrity is a new part of their security model and it’s not about to go away. Using kernel bugs to subvert it means that these measures would eventually be fixed, while exploiting 3rd party drivers potentially allows malware to figure out how to do this as well, and use the 3rd party driver maliciously. It is also a method that can be protected against, since Vista does have the ability to do per-driver blocking, and once the 3rd party vendor has upgraded all the customers, the older driver can be killed (or even shimmed against, since various kernel infrastructure in Vista allows for this kind of real-time patching).

I am currently exploring other avenues for allowing open source drivers to function on 64-bit Vista without requiring developers to pay for a certificate and deal with theÂ code signing authorities, while still respecting Vista’s KMCS policy, and continuing to protect against malicious drivers using such a method for their own gain. It is my hope to find a solution which will both please Microsoft and the KMCS policy, as well as make life easy for open source developers (and other non-commercial hobbyists) which for whatever reason don’t want to, or cannot, pay for a certificate.